The present invention relates to a motor drive control device for driving a voice coil motor for moving the magnetic head of a hard disk device or the like and an operation method thereof, and particularly to a technique which is effective in enabling calibration for improving the accuracy of detection of a counter electromotive voltage, which is for detecting the speed of a motor, when the motor drive control device is integrated in a semiconductor integrated circuit having a small chip area.
In a hard disk device (HDD), a magnetic disk is rotated at a high speed by a spindle motor. To the medium surface of the rotating magnetic disk, a read/write magnetic head is brought closer and moved in the radial direction of the magnetic disk by a voice coil motor to write/read information to/from the magnetic disk.
Also, in the hard disk device (HDD), a load/unload method is used in which, when data reading/writing is not performed, the magnetic head is retracted into a lamp mechanism located outside the outer circumference of the magnetic disk and stopped therein. By moving the magnetic head from the retracted position in the lamp mechanism to the medium surface of the disk by a load operation according to an instruction from a host, a read/write operation is performed. After the read/write operation is completed, by an unload operation according to an instruction from the host, the magnetic head is reversely moved from the medium surface of the disk to the retracted position in the lamp mechanism.
Meanwhile, with an increase in the recording density of the hard disk device (HDD), the amount of float of the magnetic head over the medium surface of the disk decreases, resulting in a demand for improved accuracy in controlling the speed of the magnetic head during the load operation. The speed of the magnetic head during the load operation is the moving speed of the magnetic head from the retracted position in the lamp mechanism to the medium surface of the disk. In conventional practice, a counter electromotive voltage generated in the voice control motor during the movement is detected and, based on the detected counter electromotive voltage, the speed of the magnetic head during the load operation is controlled.
In FIG. 13 of Patent Document 1 shown below and the disclosure related thereto, it is stated that, by amplifying the voltage between the both two terminals of the coil of the voice coil motor and the voltage between the two terminals of a sense resistor coupled in series to the coil using a two-stage feedback differential amplifier, the counter electromotive voltage in the voice coil motor is detected. However, a circuit for detecting the counter electromotive voltage is a dedicated hardware circuit, leading to the problem that a temperature-induced change in the resistance value of the coil of the voice coil motor is large and an accurate detected value of the counter electromotive voltage cannot be obtained. To solve the problem, in FIGS. 1 and 2 of Patent Document 1 shown below and the disclosure related thereto, it is stated that an analog/digital converter (ADC) and a CPU are used. That is, the voltage between the both two terminals of the coil of the voice coil motor and the voltage between the both two terminals of the sense resistor coupled in series to the coil are converted by the analog/digital converter (ADC) to digital values. From the digital values of the voltage between the two terminals of the coil and the voltage between the two terminals of the sense resistor, the CPU calculates the counter electromotive voltage and detects the speed of a head actuator.
In each of Patent Documents 2 and 3 shown below, it is stated that, in a voice coil motor drive circuit in a hard disk device (HDD), the seek operation of a magnetic head and the track follow operation thereof are performed by PWM drive to solve the conventional problems of two methods which are the PWM drive during the seek operation and linear drive during the track follow operation, such as design loads and circuit scales. Note that, as has been well known, the seek operation is the operation of moving a magnetic head to a desired storage track and the track follow operation is the operation of causing the magnetic head to follow the desired storage track for a read/write operation. It is also stated in each of Patent Documents 2 and 3 shown below that a control circuit for the feedback control of a drive current in the coil of a voice coil motor in the hard disk device (HDD) is formed of a digital circuit. Additionally, in each of Patent Documents 2 and 3 shown below, a counter electromotive voltage estimation circuit is also described which calculates a counter electromotive voltage from a drive voltage command signal as an input to a ΣΔ modulator for driving an output driver for driving the coil of the voice coil motor and from a parasitic resistance in the coil. It is stated that the counter electromotive voltage estimation circuit includes a register for holding the value of the parasitic resistance in the coil and the value of a power source voltage for the control circuit, a multiplier, and an adder/subtractor.
In Patent Document 4 shown below, it is stated that, to allow an increase in the accuracy of magnetic head positioning control and a reduction in seek time during tracking, a linear drive mode is implemented in which a drive current in a voice coil motor is subjected to analog control when the amount of movement/drive of a magnetic head is small, while a pulse drive mode (PWM) is implemented in which the drive current is subjected to digital control when the amount of movement/drive thereof is large. In the linear drive mode, a voice coil motor drive circuit outputs the drive current in accordance with a detection output of an error detection circuit for detecting the difference between a current detection voltage obtained by converting the drive current in the voice coil motor to a voltage and a control command value. By contrast, in the pulse drive mode, an output current from the voice coil motor drive circuit is subjected to PWM control by a PWM circuit for generating a pulse having a pulse width which changes in accordance with the detection output of the error detection circuit. Selection between the two drive modes is implemented by a mode change switch having a first input terminal and a second input terminal respectively coupled to the non-inverting input terminal of the PWM comparator of the PWM circuit and the output terminal thereof. In response to a LOW-level drive mode change signal, the mode change switch selects the output terminal of the PWM comparator as the second input terminal so that the pulse drive mode (PWM) is implemented under the PWM control in which the pulse width changes. In response to a HIGH-level drive mode change signal, the mode change switch selects the non-inverting input terminal of the PWM comparator as the first input terminal so that the linear drive mode is implemented in which the voice coil motor drive current is subjected to continuous feedback control.